A filter unit has an important function to fulfil in today's society since polluted air is generated in many places. Such air can be supplied to the inlet opening of a filter unit, pass through it, and shed all impurities so that clean air flows out of the outlet of the filter unit and can be mixed with air in the normal atmosphere. The filter in a filter unit can be constructed in many different ways. It may be constructed mechanically and consist of various layers of material that permit diffusing. The problem with existing filter units is that they require frequent servicing and service is expensive. It is therefore desirable to provide a filter for which the service intervals are as long as possible.
The object of the present invention is to provide a filter unit that forms part of an industrial plant and where maximal intervals are obtained between servicing. The filter unit has an inlet opening and an outlet opening. The outlet opening is connected to an extractor that produces negative pressure. The inlet opening of the filter unit is connected, via a connection unit, with a unit that generates polluted air. In accordance with the invention a number of part-filter units are stacked one above the other so that a series-connected filter is obtained. Each part-filter comprises a number of filter mats located parallel with the flow direction of the air and also with each other. The individual mats are spaced from each other and are suitably formed from a single fibre mat that is pleated. A mat consists of an intermediate layer and a surface layer on each side of the intermediate layer. The surface layer consists of fibres that are thermally joined and form permeable openings for air containing particles. Between said surface layers is an intermediate layer suitably having a thickness of between 10 and 20 mm The intermediate layer consists of several layers of fibres and its diameter may be between 5 and 15 μm. The intermediate layer may have a density of 100-200 kg/m3. Polluted air can pass through such a part-filter unit with fibre mats in the direction of the stacked part-filter units. When flowing air enters a part-filter unit it is arranged to flow through a mat. The filtering property obtained through flowing air is dependent on the speed at which it passes through the mat and the density of the mat. It has been found that if the flow velocity through the mat is lower than preferably 0.1 m/s the particles to be sorted out will fall down against the flow direction so that the part-filter unit becomes substantially self-cleaning. The filtering action is thus dependent on the flow velocity through the mat, the density of the mat and the size of the fibres. The fibres in the mat may be pinned.
In certain cases it may be advisable to have an intermediate layer consisting of two layers, in which case one layer may be of the same type as that described above. The additional layer has fibres that are randomly oriented and suitably have a diameter of 0.5-3.5 μm. The layer may have a thickness of about 5-10 mm and a density suitably lying between 20 and 30 kg/m3.
Each of the various part-filter units in a stack is intended for particles of a predetermined size. The number of part-filter units desired is determined by how polluted the air is. The part-filters stacked one on top of the other can together supplied air that is absolutely free from particles.
The part-filter located nearest the exit of the filter unit is a security unit for the entire filter unit. In this filter unit the fibre mats consist of a HEPA mat which guarantees filtration of 99.97% and filters particles with a diameter of down to 3 μm. This part-filter unit thus guarantees that the total filter unit will function if any of the previous part-filters fails to function. If the previous part-filters function perfectly there will be extremely low load on the filter nearest the exit from the filter unit.
The velocity of polluted air from operating machines should be adjusted by the extractor to 20 m/s in order to prevent impurities becoming lodged on the transfer unit. A collecting unit is arranged at the inlet to the filter unit to collect liquid and heavy particles. A velocity of 6 m/s is suitable for the air flowing into the inlet opening of the stacked part-filter units.
The filter mats in a part-filter unit may be arranged in a tubular magazine that is open at the top and bottom and forms a unit that is easily replaceable when servicing is required.
The extractor arranged at the outlet end of the filter unit may be built into the filter unit to form a single unit or it may be arranged separate from the filter unit.
The extractor, filter unit and transfer means should constitute a hermetically sealed unit.
To obtain the correct speed a number of sensors are placed at various points in a complete plant and connected to a control unit which in turn regulates the air flow.
The air flow through a filter mat is controlled by the negative pressure prevailing on the side where the air leaves the mat after having passed through it.
Additional characteristics of the present invention are revealed in the following description describing one embodiment of the invention and in the appended claims.